GULF OIL SPILL: Oil fouls the water near nesting pelicans on an island in Barataria Bay just off the the coast of Louisiana.

AP

PELICAN COVERED IN OIL: An oiled bird on an island in Barataria Bay just off the the coast of Louisiana.

Reuters

CONTAMINATION: CNN reporter Anderson Cooper lays down on the bow of an air boat to take a picture as Governor Bobby Jindal, right, removes a fishing net from the oil contaminated water in Pass A Loutre near Venice, Louisiana.

EFFECTS: Marine reef ecologist Scott Porter holds barnacle samples he removed from an oil rig in waters in the Gulf of Mexico. Porter plans to determine the effect of oil from the Deepwater Horizon spill.

AP

Marine reef ecologist Scott Porter works to remove oil from the Deepwater Horizon spill off his hands.

Reuters

GULF SPILL: Oil-tainted water from the Gulf of Mexico sits in jars placed before a group of business people and officials called Gulf Voices as they speak of their plight during a visit to Capitol Hill in Washington.

The long-awaited trial of BP for the biggest US offshore oil spill has begun, with governments, businesses and individuals blaming the company for the 2010 disaster that killed 11 rig workers and spilled four million barrels of oil into the Gulf of Mexico.

"The primary fault for this disaster lies with BP," Assistant US Attorney Mike Underhill said at the start of the trial over legal culpability for the Deepwater Horizon spill.

The trial at the federal district court in New Orleans will be overseen by Judge Carl Barbier with no jury.

Lawyers for other plaintiffs also slammed executives for BP, as did attorneys for two of BP's main co-defendants. BP's lawyer was due to respond later.

BP must show its mistakes do not meet the legal definition of gross negligence required for the highest amount of damages.

Beyond that, potential liabilities stretch into the tens of billions of dollars if Barbier determines BP or the other defendants were grossly negligent.

Oil came ashore from Texas to Florida, threatening livelihoods and state economies dependent on seafood and tourism, so the list of plaintiffs is long.

Most observers still expect the case to be settled before the trial results in a verdict.

Underhill said that less than an hour before BP's long-troublesome Macondo well ruptured and caused an explosion, BP's top well site leader on the rig called an engineer in Houston to discuss a critical pressure test that indicated problems.

Company officials did not stop the operation and "11 souls had 47 minutes to live the rest of their lives," Underhill said in his opening arguments after a weekend of talks produced no last-minute settlement.

Underhill said the accident could have been avoided if onshore engineer Mark Hafle and well site leader Don Vidrine on the rig had done their jobs.

Vidrine also faces criminal charges in the disaster, as does Robert Kaluza, the other highest-ranking supervisor aboard the rig before the disaster.

Jim Roy, an attorney for other plaintiffs suing well owner BP, rig owner Transocean, cement services provider Halliburton Co and others, said BP executives at the highest level felt pressure to push output to the limit.

"Production over protection. Profits over safety," said Roy, who represents plaintiffs who did not take part in an US$8.5 billion settlement BP struck last year.

Roy also said Transocean opened the door to disaster with poor staff training and poor maintenance of seabed equipment, while Halliburton made substandard cement to plug the well.

Transocean's lawyer, Brad Brian, also came out swinging against BP, saying rig workers trusted the oil company and died betrayed.

Brian said the inaction following the phone call showed Hafle and Vidrine did what they and others at BP had been doing for two months in the face of a risky well: "They did nothing."

Brian noted they made these decisions despite the fact that BP employees called Macondo the "well from Hell" in emails.

Halliburton's lawyer, Don Godwin, made similar arguments about BP but also said Transocean's rig crew should have shut in the well at the first sign of trouble.

"Now is when they want to pass the buck and blame my client for their misdeeds," he said.ASSESSING BLAME

Barbier, the judge overseeing the trial, has deep roots in the Gulf Coast. Born in New Orleans in 1944, he attended Southeastern Louisiana University and Loyola University New Orleans School of Law.

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ENVIRONMENTAL ANGER: Activists protest in front of the Hale Boggs Federal Building on the first day of the BP trial in New Orleans.

He was a lawyer in private practice for many years in New Orleans before President Bill Clinton tapped him for the federal bench in 1998.

The judge, who has also handled several high-profile cases stemming from Hurricane Katrina, postponed the trial date by more than a month.

An army of media have descended on New Orleans to cover the trial, and the delay avoided a clash with the NFL Super Bowl in New Orleans on February 3 or the city's Mardi Gras festival on February 12.

The fact that the case has not yet settled surprises many. "I never thought that they intended to try this case and really cannot afford to do so because the exposure is too potentially catastrophic," said Blaine LeCesne, a professor at Loyola University College of Law in New Orleans.

The trial's first phase focuses on how much each company is to blame and the degree of negligence.

Luther Strange, Alabama's attorney general, said he would seek to show all three companies had acted with "gross negligence and wilful misconduct."

"We will ask the court at the end of this trial to rule that all three - BP, Transocean, and Halliburton - are liable for punitive damages to the state of Alabama," Strange said.

Louisiana Attorney General Buddy Caldwell said his state was suffering with more than 212 miles (341km) of coast still being polluted by oil "less than 30 miles 48km) from the door of this courthouse."

Any punitive damages would come on top of billions in potential fines under the Clean Water Act.

The payout by BP so far included a record US$4.5 billion in penalties, and a guilty plea to 14 criminal counts to resolve charges from the Justice Department and civil claims from US securities regulators.

BP has sold assets to help cover its spill-related costs, including its older, smaller Gulf of Mexico operations.

The second phase of the trial, expected to start in September, will focus on the flow rate of the oil that spewed from the well. The third phase in 2014 will consider damages.